Single phase, three phase and DC — instant voltage drop calculations for electricians and designers. AS/NZS 3000 · NEC · IS 732 · SANS 10142
Standard residential and commercial single phase circuits — lighting, power points, appliances.
Three phase circuits — enter line-to-line voltage and total three phase load.
12V, 24V, 48V LED lighting, solar and battery systems. Voltage drop is critical in low-voltage DC systems.
Voltage drop is one of the most important checks in any electrical installation. Whether you're sizing a sub-board feed, designing a lighting circuit, or wiring a solar battery system, understanding and calculating voltage drop correctly ensures equipment operates correctly and installations comply with AS/NZS 3000 and other relevant standards.
Every conductor — copper or aluminium — has electrical resistance. When current flows through that resistance, energy is lost as heat, and the voltage at the far end of the cable is lower than at the source. This is described by Ohm's Law: V = I × R, where V is the voltage drop in volts, I is the current in amps, and R is the total resistance of the cable. For a two-wire circuit (active and neutral, or positive and negative), total resistance is twice a single conductor's resistance — which is why the factor of 2 appears in the standard formula.
Where: L = one-way cable length (m), I = current (A), ρ = resistivity (Ω·mm²/m), A = cable cross-section (mm²). Three phase uses √3 (1.732) instead of 2 because the three phases partially cancel each other out.
| Material | Resistivity ρ (Ω·mm²/m) | Relative to Copper | Common Use |
|---|---|---|---|
| Copper | 0.0175 | 1.0× | All sub-circuits, standard wiring |
| Aluminium | 0.028 | 1.6× | Large mains feeders, overhead lines |
| Gold | 0.022 | 1.26× | Connectors only (cost-prohibitive for cables) |
| Silver | 0.016 | 0.91× | Specialist/high-performance applications |
| Region | Standard | Max Voltage Drop |
|---|---|---|
| Australia / New Zealand | AS/NZS 3000 | 5% (3% recommended for sensitive loads) |
| United States | NEC (informational) | 3% branch circuit, 5% total |
| India | IS 732 | 5% (3% for lighting) |
| South Africa | SANS 10142 | 5% |
| United Kingdom | BS 7671 | 3% lighting, 5% other |
| Europe | IEC 60364-5-52 | 3% lighting, 5% other |
In Australia, AS/NZS 3000:2018 specifies that voltage drop from the point of supply to any point in the installation shall not exceed 5% of nominal supply voltage — that's 11.5V for a 230V single phase circuit, or 20V for a 400V three phase circuit. Many designers work to a tighter 3% limit to allow for future load growth and to account for supply voltage variations of ±6% from nominal. Some sensitive equipment such as VFDs and medical devices specify even tighter tolerances.
Four variables determine voltage drop: current (I), length (L), cable cross-section (A), and resistivity (ρ). Of these, the designer typically controls cable size and routing. To reduce voltage drop, you can:
A common mistake is to assume that a cable sized for ampacity is automatically adequate for voltage drop. This is not always true. For long cable runs — particularly in industrial, agricultural and rural settings — voltage drop is often the more limiting factor. Both checks must be performed independently:
Always take the larger cable size from both checks. The cable comparison table in the calculator above shows voltage drop for all cable sizes simultaneously, making it easy to identify which size satisfies both requirements.
Electric motors draw a large inrush current when they start — typically 5 to 7 times full load current for direct-on-line (DOL) starters. This inrush lasts 2–10 seconds and causes a momentary voltage dip on the supply. Motor starting voltage drop must be checked separately from running voltage drop, as the currents involved are much larger.
| Starting Method | Starting Current (× FLC) | Notes |
|---|---|---|
| Direct-on-line (DOL) | 6–7× | Highest inrush, simplest control |
| Star-Delta | 2–3× | Reduces starting torque too |
| Soft Starter | 2–4× | Programmable ramp time |
| Variable Frequency Drive (VFD) | 1–1.5× | Best control, highest cost |
| Autotransformer | 2–4× | Used for large motors |
Industry practice is to limit starting voltage drop to 5% at the supply terminals to avoid affecting other loads. Motor torque varies with the square of voltage — a 10% drop reduces torque by approximately 19%.
DC systems — including solar battery installations, 12V/24V LED lighting, EV charging and telecommunications — are particularly susceptible to voltage drop because the system voltage is low. A 1.2V drop on a 230V AC circuit is just 0.5%, but on a 12V system it represents 10% — significant enough to cause LED lights to flicker, solar charge controllers to malfunction, or battery charging to be incomplete. Practical rules of thumb:
For a full standalone voltage drop calculator with 5 modes including motor starting and series runs, visit voltagedrop.com.au.
Scenario: A 2,400W split-system air conditioner is installed 25 metres from the main switchboard. Supply is 230V single phase. Cable is 2.5mm² copper TPS. Power factor 0.95.
Step 1 — Find current: I = P / (V × PF) = 2,400 / (230 × 0.95) = 10.98 A
Step 2 — Apply formula: VD = 2 × 25 × 10.98 × 0.0175 / 2.5
Step 3 — Result: VD = 3.84 V | VD% = 3.84 / 230 × 100 = 1.67%
✅ PASS — 1.67% is well within the AS/NZS 3000 limit of 5%. 2.5mm² is adequate.
Scenario: A 22kW three-phase motor is supplied from a sub-board 80 metres away. Supply is 400V three phase. Cable is 6mm² copper. Full load current 40A. Power factor 0.86.
Step 1 — Three-phase formula: VD = √3 × L × I × ρ / A
Step 2 — Calculate: VD = 1.732 × 80 × 40 × 0.0175 / 6 = 16.13 V
Step 3 — Check %: VD% = 16.13 / 400 × 100 = 4.03%
⚠️ BORDERLINE — 4.03% passes the 5% limit but is tight. Consider upgrading to 10mm² (VD = 2.42%) for future load growth.
Scenario: A 48V DC solar battery system feeds an inverter 12 metres away. Battery cable carries 60A through 16mm² copper. Maximum allowable voltage drop is 3%.
Step 1 — DC uses same formula as single phase: VD = 2 × L × I × ρ / A
Step 2 — Calculate: VD = 2 × 12 × 60 × 0.0175 / 16 = 1.58 V
Step 3 — Check %: VD% = 1.58 / 48 × 100 = 3.28%
⚠️ SLIGHTLY OVER 3% — Upgrade to 25mm² (VD = 1.01V, 2.10%) to meet the 3% design limit.
Scenario: A 15kW DOL motor (full load current 28A) starts on a 400V three-phase circuit. Cable is 6mm² copper, 30m. DOL starting current is 7× FLC = 196A.
Running VD: VD = 1.732 × 30 × 28 × 0.0175 / 6 = 4.24 V (1.06%)
Starting VD (196A inrush): VD = 1.732 × 30 × 196 × 0.0175 / 6 = 29.7 V (7.42%)
Ampacity check: 6mm² copper is rated ~38A — adequate for 28A FLC.
✅ PASS — 7.42% starting VD is within the 15% industry limit. Check that sensitive equipment on the same supply is not affected by the momentary dip.